As electrical devices and systems have become increasingly prevalent in consumer and industrial applications, there has been a corresponding increase in the use of batteries. The uses of batteries to supply electrical power are as varied as the electrical devices or systems in which they are used. Some electrical systems, such as portable electronic devices, use batteries as their primary source of electrical energy. Other electrical systems or devices receive their primary supply of electrical power from a power source such as a generator, power plant, or line power supply. Even these devices often utilize a battery, however, as a back-up or secondary supply of electrical power. The method of the present invention is intended for use in electrical systems using a battery as a back-up or secondary power supply (herein "battery-backed" systems). In such battery-backed systems, if the primary power source fails, the battery can be used to supply electrical power until the primary power supply is reinstated. This scheme of redundant power sources is often utilized in electrical devices or systems in which a temporary loss of power is problematic. Such systems include very complex as well as relatively simple applications. Examples include: alarm clocks, where a loss of power could result in the clock losing track of the proper time thus resulting in a false or a late alarm; computers, where an untimely loss of power could result in lost data; and telecommunications systems, where a loss of power could result in a shutdown of communications networks.
Regardless of the electrical system in which it is used, a battery is simply a device used to store electrical energy. As used herein, the term battery will include both a singular device used to store electrical energy as well as multiple storage devices connected in an array or other configuration to provide additive storage capacity. The process of storing electrical energy or power into a battery is referred to as charging or recharging the battery. Conversely, the process of removing or using the stored electrical energy from a battery is referred to as discharging the battery. The amount of electrical energy stored in a battery is typically referred to as a battery's capacity (Q) and is measured in units of ampere-hours (AH). The unit ampere-hours is indicative of the relationship between a battery's remaining capacity (Q), reserve time (t), and the current (I) being supplied by the battery. Specifically, the relationship corresponds to the following equation: ##EQU1## As indicated by this equation, there is an inverse relationship between reserve time (t) and current (I). That is, the greater the current being supplied by the battery, the faster the battery discharges its stored capacity of electrical energy, and thus, the shorter the time the battery can supply such current before completely discharging its total capacity. Conversely, the smaller the current supplied, the slower the battery discharges, and the longer the battery can supply such current before becoming completely discharged.
The total amount of energy that can be stored in a battery, i.e. a battery's total capacity, depends on the type, size, and condition of the battery. Since a battery can only store a limited amount of electrical energy, once that energy has been exhausted the battery will no longer be able to supply electrical power to the electrical system or device. Obviously then, for any electrical system incorporating a battery, knowing how much battery capacity remains is a convenient feature since a battery's remaining capacity determines the battery's reserve time, i.e., how much longer before the battery supply is exhausted and thus how much longer the electrical device or system may be used. In electrical systems which require an uninterrupted power supply, determining when the battery power supply will be exhausted may not only be a convenient feature but such capability may be a critical system design feature. In order to ensure an uninterrupted power supply, the remaining battery capacity or reserve time needs to be accurately predicted such that either the primary power supply can be restored to service, or another alternative power supply can be connected, before the battery power supply is exhausted.
In many systems that utilize a battery, the system is specifically designed with the capability of monitoring the condition or health of the battery. Some systems incorporate a capacity indicator, or "fuel gauge," which shows the available battery capacity. Such a fuel gauge allows one to determine if the battery has sufficient capacity to support the system for a sufficient time before the primary power is reinstated. Moreover, such a fuel gauge can be useful during a battery discharge to determine how much battery capacity remains and thus how much reserve time the battery has left.
Various diagnostic methods and apparatus have been developed to monitor the condition of a battery and to provide an estimate or prediction of the battery's performance characteristics; i.e., remaining capacity (Q) and reserve time (t). Typically, these methods utilize some combination of predetermined battery parameters, which are indicative of the battery's expected or ideal performance, and measured battery parameters, which characterize the battery's actual performance during a battery discharge. Using these parameters, a prediction of the battery's remaining capacity (Q) and/or reserve time (t) can be provided with varying levels of accuracy.
A majority of the available methods and apparatus for predicting battery performance, however, only do so for a discharging battery. In fact, most battery diagnostic methods only contemplate monitoring the battery during a battery discharge and providing a prediction of battery condition and performance based on measurements of the battery's performance during the discharge. Herein, we will refer to these methods generally as "discharge diagnostic methods" because they predict battery reserve time or capacity based on measurements of battery performance during a battery discharge. The reason most battery diagnostic methods are "discharge diagnostic methods" is because a battery's capacity and reserve time can be much more accurately predicted using data from a battery discharge. The reason for this is simple: the battery is really only in use when it is discharging, that is, when it is providing the electrical power for the system. By measuring or monitoring the battery's performance while it is in use (i.e., during a discharge), the battery's actual condition can be determined. Without knowing the battery's actual condition, assumptions about the battery's condition would have to be made in order to predict the battery's future performance. Without a discharge, the battery is typically assumed to be performing as a new battery of the same type, ignoring the effects of aging, temperature, environment, etc. Whenever such assumptions are made the accuracy of any resulting predictions suffers.
Examples of available methods and apparatus for predicting battery performance include the following:
The initial diagnostic methods used for predicting remaining battery capacity or reserve time were strictly empirical, wherein extensive testing of the battery would be conducted in order to compile a large database of characteristics indicative of the battery's performance throughout the cycle of the battery from a fully charged state to a fully discharged state. By comparing these predetermined test characteristics to the battery's actual characteristics, as measured during use, one could predict what stage of discharge the battery was in and thus how much battery capacity or reserve time remained.
For these empirical methods to yield accurate and reliable results, however, the initial testing had to account for a multitude of factors which could affect the battery's performance. This means the testing had to be performed under conditions matching the actual use of the battery as closely as possible. Not only did this mean testing had to be performed for each type and size battery individually, but also the testing needed to include other external variables such as the load on the battery as well as the battery's temperature and environment (all factors which would affect the battery's performance characteristics). The result is that there were innumerable combinations of such factors which would have to be tested for each battery in order for the empirical data to be useful and accurate for all applications. Moreover, to have test data useful for reliably predicting a specific battery's performance essentially required duplicating the application in which the battery was going to be used. This was obviously impractical to do for all possible applications. Typically, then, the testing would be standardized by performing the tests with standard loads and standard variables for the surrounding temperature/environment for each of the different types and sizes of batteries. The data from these standardized tests, however, provided limited accuracy and reliability for predicting the remaining battery capacity and reserve time.
Other more theoretical-based diagnostic methods have been utilized to address the inherent limitations of attempting to rely strictly on such empirical methods for predicting the remaining capacity and reserve time of a battery. One such method of prediction is based on the Peukert equation: EQU t=aI.sup.b
where (t) is the reserve time to a given end voltage, (I) is the discharge current and (a) and (b) are empirically determined parameters. The remaining reserve time during discharge is obtained by subtracting the actual time of discharge from the value (t) given by the equation. The only real time data used in this approach is the discharge current (I), while the parameters (a) and (b) are experimentally predetermined by extensive testing, data acquisition, and parametric analysis. Since these parameters are empirically derived, the values of these parameters are fixed and do not adapt to changing conditions affecting battery performance such as changing load requirements, temperature, or aging of the battery.
An attempt to provide more accurate predictions by being more responsive to changes in battery behavior during discharge is disclosed in the patent application Ser. No. 08/013,272, filed Feb. 4, 1993, submitted by D. Levine et al. now U.S. Pat. No. 5,371,682 which utilizes matrices of predetermined parameters that correlate the slope of the voltage versus discharge time at various discharge currents, battery voltages during discharge, and end voltages. The use of voltage-versus-time slopes for prediction allows the method to be highly adaptable to changes in battery behavior during discharge. This method, however, also requires extensive initial testing to derive the data to populate the matrices.
Another discharge diagnostic method is disclosed by R. Biagetti and A. Pesco in U.S. Pat. No. 4,952,862. This method operates by measuring the difference between battery voltage during discharge and the battery plateau voltage, EQU V.sub.battery -V.sub.P.
During discharge this difference is plotted against a ratio of discharged capacity to the total discharge capacity available: EQU Q.sub.removed /Q.sub.to-end-voltage.
This plot, created from measured data, is a single curve having an exponential and a linear region. The curve can then be used to determine remaining capacity and reserve time from the measured discharged capacity (Q.sub.removed) and the plateau voltage (V.sub.p).
Another approach in determining the reserve time of a discharging battery, disclosed in U.S. Pat. No. 4,876,513, takes advantage of the fact that when battery voltages (corrected for internal resistance) are plotted versus a ratio of ampere-hours remaining to ampere-hours available to a certain discharge voltage, all discharge curves fall on a single curve. The battery voltages are calculated using a battery internal resistance that is measured periodically during discharge.
Although moderately effective, none of these preexisting methods for evaluating the state of a discharging battery works accurately at all temperatures, requires only a minimal number of empirically derived parameters, is independent of the battery size being monitored, and adapts to changing conditions affecting battery performance. In response to these deficiencies, Trung V. Nguyen developed a more accurate apparatus and method of predicting remaining battery capacity (Q) and reserve time (t) of a discharging battery to a selected end voltage. The method is disclosed in U.S. Pat. No. 5,631,540 and is primarily based on measurable battery parameters which do not require extensive pre-testing of the battery. The Trung method is the preferred discharge diagnostic method used for a portion of the inventive method of the present invention. Accordingly, the description of this method in U.S. Pat. No. 5,631,540 is incorporated herein in its entirety.
In the Trung method, the battery reserve time (t) of a discharging battery is determined by an arrangement considering the discharge current (I), battery voltage (V), battery temperature (T), and the battery's internal resistance (R.sub.int). The remaining battery capacity (Q) is determined from the ratio between a maximum theoretical capacity (Q.sub.max) and its present capacity (Q). A term defined by a sum of the battery full charge open circuit voltage (E.sub.ac) and the voltage loss due to the internal resistance of the battery (IR.sub.int) and the battery voltage on discharge (V) divided by the battery temperature (T), is computed as the temperature-corrected battery overvoltage (.eta.): ##EQU2##
The characteristics of the battery discharge are reduced to a ratio of the remaining battery capacity to maximum theoretical capacity: ##EQU3## This normalized battery capacity value is plotted versus the temperature-corrected battery overvoltage to produce a discharge characteristic curve that is invariant to discharge rates, temperatures, and battery size. This normalized battery capacity is determined by fitting parameters to the overvoltage value .eta. by the relation: ##EQU4## to characterize the discharge characteristic and determine Q. A reserve time (t) can then be calculated from the determined capacity value (Q) using the relation: ##EQU5## The characteristic curve and the dynamic variables can be stored in a computer and processed continuously to provide a continuing real time prediction of the remaining capacity (Q) and reserve time (t) of the battery on discharge.
Ultimately, however, it should be understood that all discharge diagnostic methods, which rely on a battery discharge to provide estimates or predictions of battery status, are inherently deficient. If no battery discharge occurs or if no discharge has occurred in some time, discharge diagnostic methods may be unable to provide an accurate prediction or even any prediction at all. Moreover, any prediction available may be stale (i.e., based on old data) and therefore inaccurate and unreliable. Because a battery's performance changes over time, ideally the battery should be monitored on an ongoing basis. For discharge diagnostic methods, which require a battery discharge, this means battery discharges must occur in frequent intervals in order to accurately monitor the condition of the battery over time. In addition to a frequency requirement, the battery discharges must be of a certain duration in order to provide enough time to collect sufficient data on the battery's performance to perform the necessary battery diagnostics. In practice, however, normal battery discharges in a system may be infrequent and/or of insufficient duration to adequately monitor the condition of the battery. In fact, battery-backed systems, which operate from a primary power source and use a battery for back-up power, may not use the battery for days, months, or even years. In such systems, by the time a sufficient battery discharge occurs to test the battery, the battery may already be defective or inadequate. Accordingly, the ability to continuously provide a relatively accurate estimate of battery condition and prediction of battery performance, before, during, and after a battery discharge, is greatly desirable.